feat: update color variables and improve styling in index.css

[mroxso]

This pull request makes significant updates to the application's base styling in src/index.css, focusing on modernizing the color palette, typography, and shadow system for both light and dark themes. It also introduces improved usage of Tailwind's layering and utility features for more consistent styling across the app.

Theme and Palette Updates:

• Overhauled the color variables for both light and dark themes, shifting from blue/purple-centric hues to a more neutral and warm palette. This affects background, foreground, card, popover, primary, secondary, accent, and chart colors, making the UI feel less cold and more inviting.
• Updated the shadow system to use more subtle, modern values and changed the color model from HSL to OKLCH for improved visual consistency and accessibility.

Typography and Utility Improvements:

• Replaced legacy font stacks with system-based, cross-platform font families for --font-sans, --font-serif, and --font-mono, enhancing readability and performance.
• Added Tailwind's base, components, and utilities inside a custom @layer base block, ensuring better layering and easier overrides for global styles.
• Applied global utility classes to all elements and the body for consistent border and background